The invention relates to a catalyst having an improved stability against deactivation at high temperatures and high partial pressure of steam, which is especially suited for steam reforming processes and methanation processes, as well as a process for substitute national gas production.
Substitute Natural Gas (SNG) can be produced in large scale from coal via gasification and subsequent methanation of the produced synthesis gas in one or several reactors to achieve sufficiently high CH4 content in the final product. The methanation step is often carried out in a series of adiabatic, fixed bed reactors, where the main reactions taking place are:CO+H2O═CO2+H2  (1)CO+3H2═CH4+H2O  (2)
In the case of methanation from CO2 and H2, it is believed that the mechanism of the reaction goes first via reverse water gas shift (i.e., the reverse of reaction (1)), followed by CO methanation to form CH4, so that the overall reaction is:CO2+4H2═CH4+2H2O  (3)
The methanation of synthesis gas is highly exothermic, which results in a large temperature increase in these reactors. Suitable catalysts for methanation thus need to be sufficiently active at low temperatures, resistant against sintering at high temperatures and high partial pressure of steam, and resistant to other deactivation phenomena, for example carbon formation. The catalyst sintering stability is most critical in the upstream reactors where the exit temperatures are the highest. The methanation process is typically carried out at elevated pressure (above 10 barg, potentially up to more than 100 barg) and at maximum temperatures between 500° C. and 750° C. with a partial pressure of steam between 2 and 15 barg, but potentially up to 30 barg.
The reverse process steam and/or oxygen reforming occurs under similar conditions in the presence of methane (and/or other hydrocarbons) and water.
A further high temperature process for which stabilization of catalyst and catalyst support is important is catalytic combustion of fuels, which may occur elevated pressures, at temperatures between 600° C. and 1000° C., and with water as a product in the case of fuels comprising hydrogen, e.g. according to (4) below.CH4+2O2═CO2+2H2O  (4)
In WO2011/087467 a catalyst comprising nickel on a support comprising alumina, zirconia and various combinations of cerium, praseodymium, and neodymium are described as reforming catalysts for fuel cells. The composition of the catalyst is not stated, but the nickel content is estimated to be above 60%, and the focus of the application is on the pore structure of the catalyst, which may be especially relevant for maintaining a well dispersed nickel phase at such a high metal concentration. The application does not consider the crystal structure of alumina or the mechanical stability of the catalysts, the catalytic activity is not demonstrated and pore structure stability is only demonstrated up to 450° C.
In WO2002/087756 a reforming catalyst comprising 3.7-16 wt % nickel on a support comprising 0.1-4.1 wt % lanthania, 0.1-2.2 wt % zirconia and magnesia/alumina of an unspecified crystal structure. The combined elemental content of La and Zr is 0.7-4.1 wt % in the support, and in all examples one of the two elements is present in a concentration below 0.5 wt %. The catalytic activity for methane steam reforming of the catalyst was tested at 750° C. at an unspecified pressure, and stability was not considered.
US 2003/032554 discloses a catalyst comprising 3-12 wt % nickel, theta-alumina and less than 5 wt % (relative to theta-alumina) of a theta-alumina modifying component, typically being a combination of zirconia and a lanthanoide such as lanthanum or cerium. All examples are based on catalyst comprising 0.9-1.0 wt % La2O3 or CeO2, 2.3-2.5 wt % ZrO2 and 2.8-12.5 wt % Ni, or catalysts further comprising calcium, magnesium and/or cesium. The combined elemental content of La, Ce and Zr is 3.6 wt % in the catalyst support. The catalytic activity for methane steam reforming of the catalyst was tested at 750° C. at atmospheric pressure, and long term stability was not considered.
Therefore based on the prior art there is no indication of the importance of stabilizing the surface area of the catalyst support at high temperatures, especially in the presence of steam, nor an indication of the stabilization of transition alumina in catalyst supports as a way to obtain a stable surface area.
Well known catalysts for methanation processes contain Ni as the active phase, which provide the highest methanation activity per unit cost, on a stabilized support containing high surface area Al2O3. At high temperatures, especially in the presence of steam high surface area transition Al2O3 (e.g. χ, κ, γ, δ, η, ρ and θ-Al2O3) tends to sinter and transform towards the thermodynamically more stable α-Al2O3 phase, leading to a loss of surface area due to the collapse of the carrier and a reduction in catalyst pellet mechanical strength. The loss of surface area can be so severe that the Ni particles also sinter together, leading to a loss of catalytic activity. The reduction in the mechanical strength can be so severe that the catalyst pellets crumble into dust during operation or unloading.